Information recording apparatus and method for modulated magneto-optical recording medium

ABSTRACT

A magneto-optical data recording system comprises magnetic field pulse applying means for applying to a magneto-optic recording medium a magnetic field pulse of a polarity corresponding to code data to be recorded, and means for heating the recording medium in a pulsating manner at a predetermined frequency synchronous with the code data to be recorded. During recording, the magnetic field pulse applying means applies a magnetic field pulse of a polarity corresponding to code data along a track of the recording medium and the heating means continuously heats the recording medium in a pulsating manner at a predetermined frequency synchronous with the code data to thereby directly overwrite the code data. The magnetic field pulse applying means includes a magnetic coil driven by code data from a source of code data (data generator). The heating means includes an optical head which comprises a source of laser beam driven by a laser driver and an optical system which focuses the laser beam from the source of laser beam as a small light spot onto the recording medium.

This application is a Continuation of application Ser. No. 08/744,760,filed Nov. 6, 1996 now U.S. Pat. No. 5,726,955, which is a continuationof application Ser. No. 08/429,008, filed Apr. 26, 1995, now U.S. Pat.No. 5,579,292 issued Nov. 26, 1996, which is a continuation ofapplication Ser. No. 07/979,338, filed Nov. 20, 1992, now U.S. Pat. No.5,473,581 issued Dec. 5, 1995, which is a continuation of applicationSer. No. 07/353,602, filed May 18, 1989, now U.S. Pat. No. 5,170,383issued Dec. 8, 1992.

BACKGROUND OF THE INVENTION

The present invention relates to magneto-optical data recording systems,and more particularly to such systems which are capable of overwritingnew coded data on old data.

At present, most magneto-optical data recording systems erase old dataand then write new data, so that the disc used must be rotated at leasttwice, once for erasing the old data from the disc and once for writingnew data on the disc. This is because it is difficult to directlyoverwrite new data on the old data.

For example, in Japanese Patent Publication JP-A-52-23318, data isrecorded on a thin magneto-optic layer by the interaction of a laser anda magnetic coil for generating a magnetic field. Namely, a magneticdomain is formed at a position heated by the laser beam using a magneticfield lower than magnetic coercive force in the thin layer. In thisarrangement, a pulsated modulated laser beam and a direct currentmagnetic field are used corresponding to the coded data. According tothis method, new data cannot be directly written so long as old dataremains.

In Japanese Patent Publication JP-A-51-107121, a modulated magneticfield pulsated with coded data and a direct current laser beam are used.According to this method, new data can be directly overwritten on a discwhere old data remains. However, in this method, the laser beam isirradiated onto the recording film with a constant light power, so thatthe recording film is overheated for longer time and the shape of theformed magnetic domain is not circular, but crescent. This method isproblematic in signal processing.

Further, in Japanese Patent Publication JP-A-62-252553, a high frequencyself-oscillating magnetic field generated by a high frequencyself-oscillating circuit and a modulated laser beam pulsated with codeddata are used. Since this method uses the high-frequencyself-oscillating magnetic field, new limited data can be directlyoverwritten on a disc where old data remains, but any new data cannot beoverwritten directly and the shape of the formed magnetic domain doesnot become circular but crescent. Thus this method is also problematicin signal processing.

These conventional techniques are not suitable for overwriting any newdata on a magneto-optic recording medium where old data remainsrecorded. Further, the shape of the formed magnetic domain would becomecrescent. There are also problems in the signal processing.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a magneto-opticaldata recording system which is capable of overwriting any code datawithout causing the shape of a magnetic domain to be a crescent.

A magneto-optical data recording system according to the presentinvention is characterized by magnetic field pulse applying means forapplying a magnetic field pulse of a polarity corresponding to code datato be recorded to a magneto-optic recording medium, and means forheating the recording medium in a pulsating manner at a constantfrequency synchronous with the code data to be recorded, wherein duringrecording the magnetic field pulse applying means applies a magneticfield pulse of a polarity corresponding to code data to themagneto-optic recording medium along its track, and the heating meanscontinuously heats the magneto-optic recording medium in a pulsatingmanner at a constant frequency synchronous with the code data to therebydirectly overwrite the code data.

The magnetic field pulse applying means uses a magnetic coil driven bythe code data from a source of code data (data generator). The heatingmeans preferably uses an optical head comprising a source of laser beamdriven by a laser driver and an optical system for focusing the laserbeam from the source of laser beam as a small light spot onto themagneto-optic recording medium. Namely, the polarity of the magneticfield pulse applied by the magnetic coil is switched in accordance withthe polarity of the code data to be recorded, and laser pulsessynchronous with the frequency of a data recording clock aresuccessively emitted from the optical head while code data is directlyoverwritten onto the magneto-optic recording medium. The frequency oflaser pulses used, are for example, the data clock frequency, and theduty cycle of the period of the data clock frequencies to the pulsewidth is set to 1.0 or less. In order to record each magnetic domainseparately without being subjected to thermal influence, the duty cycleis preferably 0.7 or less. If the laser pulse width is extremely short,the intensity of the laser beam must be increased greatly. To this end,the laser pulse width is required to be expanded to some extent, and theduty cycle is required to be 0.1 or more and preferably 0.2 or more. Asjust mentioned above, laser pulses of a predetermined width phase-lockedto a data clock of a predetermined frequency are emitted from an opticalhead to heat small regions of a moving recording medium at equalintervals periodically along a track of the medium, a magnetic fieldcorresponding to the polarity of code data to be recorded, is generatedfrom a magnetic coil to directly overwrite the coded data along thetrack. Preferably, reproduction signals indicative of embedded pitspreformed at predetermined intervals periodically along the track of themagneto-optic recording medium are preferably used as the data clock.Namely, a recording medium where servo areas and data areas arealternately disposed along the track is preferably used to generate aclock signal from embedded clock pits preformatted in the servo area,and the recording of the code data is controlled based on the clocksignal. Preferably, wobbled pits for tracking signal detection arepreformatted together with the embedded clock pits in the servo area.The embedded clock pits can be reproduced by using the laser beam fromthe optical head as the heating means. In more detail, the source oflaser beam is continuously oscillated at its low output in the servoarea, and the reflected beam from the recording medium is extractedseparately from the beam from the source of laser beam using a polarizerbeam splitter provided in the optical head, and the reflected beam isdetected by a photodetector to thereby reproduce the clock pits using achange in the intensity of the reflected beam. The embedded clockingmeans that the clock is generated by preformatted clock pit. A pulsesignal corresponding to a clock pit in the servo area is extracted fromthe output of the photodetector by a clock extraction circuit togenerate a clock signal of a frequency N times the frequency of andsynchronous with the extracted signal using a PLL (Phase Locked Loop).During data recording, the laser driver is controlled by this clocksignal to thereby successively irradiate high-output laser pulsesphase-locked to the clock signal from the optical head and a magneticfield is applied by the magnetic coil and its polarity is changeddepending on the code data which is read out synchronously with theclock signal from the source of code data (data generator), whereby inthis method, both of the irradiated laser beam and the magnetic fieldare modulated synchronously with the clock signal. Since it takes aconsiderable time for the applied magnetic field to rise up and falldown compared to the case of laser pulses, a phase adjuster ispreferably provided which adjusts the relationship in phase between thelaser pulse and the magnetic field pulse such that the laser pulse isapplied under the condition where the magnetic field pulse hassufficiently risen or fallen.

According to the present invention, the laser beam and the magneticfield are modulated synchronously with the data clock signal, the laserbeam is caused to locally heat the magneto-optic recording medium atequal intervals, the polarity of the magnetic field is changed inaccordance with the code data, and a new magnetic domain is formedcorresponding to new code data by the interaction of the laser beam,magnetic field and thin magneto-optic film. Thus, the laser beam heatsthe recording medium at equal intervals in a pulsating manner, so thatthe magnetic domain does not become crescent, which is preferable forsignal processing.

When the laser, for example a semiconductor laser, is used, a pulsemodulation is performed at high output in the data recording region,which is preferable for the lifetime of the laser.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a magneto-optical disc system as one embodiment ofthe present invention;

FIG. 2 illustrates waveforms explaining the operation of the discsystem;

FIG. 3 illustrates the details of the waveform;

FIGS. 4A and 4B are each a waveform illustrating an improvementaccording to the present invention; and

FIGS. 5(A) to 5(E) and 6A to 6E illustrate problems to be solved by thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Before entering the explanation of an embodiment of the presentinvention, the problems to be solved by the present invention will benow described by taking a sampled servo method using an embedded clockas an example with reference to FIGS. 5(A) to 5(E) and 6A to 6E.

In the magneto-optical disc system using a sampled method, each oftracks 2 on a disc is divided into 1000-2000 segments 3, as shown inFIG. 5A. Each segment 3 is divided into a servo area 4 where informationto control the system is recorded beforehand and a data area 5 where theuser records code data. Formed in each servo area 4 are pits 6 fordetecting a tracking signal and an embedded clock pit 7 to generate aclock for a data signal. In order to control the flow of the code datasignal, a data channel clock 9 (FIG. 5(C)) is generated by a PLL from asignal 8 (FIG. 5(B)) comprising the reproduction of an embedded clockpit 7 using the laser beam. The clock 9 is, for example, about 10 MHz.According to the system disclosed in the JP-A-52-23318, when code data10 is overwritten onto the data recording area 5, the laser beam ismodulated with code data 10 in the data area 5. In the case of the codedata 10, for example, (11001), on the disc, a small area in which thecode corresponding to the "1" is recorded is heated to hundreds ofdegrees °C. with the modulating light 11 (FIG. 5(D)) and magneticdomains 13 (FIG. 5(A)) are formed using a direct current magnetic field12 (FIG. 5(E)) generated by the magnetic coil. With such conventionaltechniques, a magnetic domain 13 of new data is formed in the data area5 with no old data magnetic domain 14 being erased undesirably. FIGS. 6Ato 6E illustrate a method employed by the JP-A-62-252553. According tothis method, a high-frequency alternating magnetic field 16 (FIG. 6B)self-oscillated by the clock 15 (FIG. 6A) is applied to the disc faceduring recording. At this time, magnetic domains 20 corresponding to themagnetic fields 19 facing upwardly or downwardly as shown in FIG. 6D areformed by applying laser pulses 18 synchronously with the position of apositive or negative peak of the high-frequency alternating magneticfield 16 in accordance with data to be recorded. Thus, by successivelyforming magnetic domains in the same direction, as shown in FIG. 6E, themagnetic domains 20 are superposed one on the other to thereby allow amagnetic domain 20 of any length to be recorded. However, according tothis method, the peripheral configuration of the recorded magneticdomain 20 is not smooth but like chained balls, so that the S/N ratio ofthe reproduced signal is deteriorated. The timing of irradiation of eachlaser pulse 18 is locked to the position of a positive or negative peakof the high-frequency alternating magnetic field 16 in accordance with"1" or "0" of recorded data, so that there is the problem that recordedmagnetic domains 20 are likely to remain unerased.

The present invention solves these problems. It is an object of thepresent invention to provide a magneto-optical data recording system inwhich the shape of a magnetic domain is not crescent but circular, andany code data can be easily overwritten with no unerased data portionremaining.

FIG. 1 shows a magneto-optical data recording system as one embodimentof the present invention. FIGS. 2, 3 and 4A to 4B illustrate theoperation of the system. In FIG. 1, reference numeral 1 denotes amagneto-optic disc which is rotated by a rotary mechanism (not shown)such as a rotary motor. It includes a thin magneto-optic film 21 and atransparent disc 22 on which the film 21 is formed. A protective film(not shown) is formed on the thin film 21 as needed. Light emitted by alaser 23 is focused by an optical system 24 comprising a collimeter lenswhich converts the emitted light to a collimated beam and a focusinglens which focuses the laser beam to form a light spot 25 onto the thinfilm 21 via the disc substrate 22. The reflected light from the disc 1is separated and extracted by a polarizer beam splitter 28 from thelaser beam emitted by the light source 23, the rotation of thepolarization plane caused by recorded data on the magnetic optical discis converted to the corresponding light quantity via an analyzer 39, andthe light quantity is subjected to photoelectric conversion by aphotodetector 29. Instead of the analyzer 39, a λ/2-plate and apolarizer beam splitter may be used to separate the reflection from thedisc into S and P polarization components, which are then detected bythe corresponding photodetectors, the outputs of which are subtractedone from the other, so that the difference results in a magneto-opticalsignal and the sum of the outputs results in a prepit signal. The lightspot 25 irradiated by the optical head is controlled in an auto-focusingmanner such that it is focused at all times onto the thin film 21following upward and downward swings of the disc 1, and is subjected totracking control such that the light spot is positioned at all timesonto a desired track following the eccentricity of tracks on the disc.These servo techniques are well known and will not be described further.For example, focus control is described in U.S. Pat. No. 4,561,082 inwhich a non-recording area for focusing purposes is provided in a servoarea where a deviation of the focus is detected by sampling. The focuscontrol is also described in U.S. Pat. No. 4,742,218 in which thereflected light from the disc is split into two light beams which arefocused before and after photo-detectors, a focus deviation is detectedby the difference between the outputs of both the photodetectors, afocusing lens is moved in the optical axis direction in accordance withthe degree of the focus deviation. Tracking control is disclosed in, forexample, U.S. Pat. No. 4,707,816 and U.S. Pat. No. 4,748,609 in which atrack deviation is detected by using a pair of pits wobbled right andleft with reference to the center line of the track. Of course, theoptical head comprising the laser light source 23, optical system 24,photodetector 29, etc., are arranged to be movable radially of the disc1.

A magnetic coil 27 is provided on the opposite side of the disc 1 fromthe optical head and constructed such that its magnetic polarity iscontrolled by the code data from a data generator 37 during recording.The magnetic coil 27 is integrally coupled to the optical head and asupport structure (not shown) thereof so as to be provided directlyabove the light spot on the disc to be thereby moved together with theoptical head and support structure. The magnetic coil 27 is preferably afloating type magnetic head combined with a floating slider (not shown).

The laser 23 is driven by a laser driver 26 such that it continuouslyoscillates at its low output power during reproduction and emits laserpulses at its high output power during recording. The magnetic coil 27applies to the disc a magnetic field of a polarity corresponding to codedata to be described during recording to perform reproduction andrecording.

FIG. 2 illustrates that operation and illustrates the relationshipbetween modulating pulses from the laser 23 of FIG. 1 and the polarityof the magnetic field from the magnetic coil 27.

As shown in FIG. 2A, each of the tracks 2 on the disc 1 is divided intoa plurality of (for example, 1000-2000) segment 3, each being beforehanddivided into a servo area 4 in which data to control the system isrecorded beforehand and a data recording area 5 in which the userrecords code data. Formed in each servo area 4 are a pair of wobbledpits 6 for detecting a tracking signal and an embedded clock pit 7 tocontrol the clock for the data signal. The wobbled pits 6 are equal inquantity and provided one on each side of the center line of the track2, and the clock pit 7 is provided on the center line of the track 22.If a light spot scans along the center line of the track 2 of FIG. 2(A),the photo-detector outputs a signal shown in FIG. 2(B) as an intensitychange of the reflected light. The reproduced signals obtained from thepair of wobbled pits 6 are equal in level when the light spot scansalong the center line of the track 2. If the light spot deviates fromthe center line of the track 2, one of the levels of the reproducedsignals is lowered and the other level rises to cause an unbalance, sothat a track deviation signal is obtained by detecting the difference inlevel between the reproduced signals from the pair of wobbled pits 6.Techniques to obtain a track deviation signal from such wobbled pits aredisclosed in the above mentioned U.S. Pat. No. 4,707,816 and U.S. Pat.No. 4,748,609. If 1000-2000 pairs of such wobbled pits are disposedalong the periphery of the track and track deviation signals obtainedfrom these pairs of wobbled pits are sampled at high frequency (at 30-40KHz), servo signals over the entire zone (DC-3 KHz) necessary fortracking can be obtained. The code data is recorded and reproduced inthe data area 5 while controlling the position of irradiation of thelight spot such that it scans the center line of the track 2 using theservo signals. The tracking servo techniques are well known and omittedin FIG. 1.

A process for obtaining a clock signal from the embedded clock pits 7will now be described. A pulse signal 8' indicative of the position ofthe peak of a signal 8 corresponding to a clock pit 7, namely, thecentral position of the clock pit 7, is obtained from the reproducedsignal shown in FIG. 2(B) (FIG. 2(C)). This signal can be easilyobtained by differentiating the reproduced signal in FIG. 2(B) anddetecting its zero point. This pulse signal 8' is extracted by anembedded clock extraction circuit 35, and a data clock (FIG. 2(D))synchronous with the signal 8' which has a frequency N times that of thesignal 8' is generated using the PLL 36. The technique for obtaining adata clock from embedded clock pits is disclosed in detail in U.S. Ser.No., now U.S. Pat. No. 4,949,325 filed by some of the inventors.

The present invention is characterized in that both of the irradiatedlaser beam and applied magnetic field are modulated synchronously withthe data clock 9. The data generator 37 is controlled in accordance withthe data clock 9, the magnetic coil 27 is caused to produce a magneticfield pulse of a polarity corresponding to the code data read from thedata generator 37 synchronously with the data clock, which is also usedto control the laser driver 26 to cause the optical head to continuouslyirradiate onto the disc high-output laser pulses phase-locked to thedata clock during data recording to thereby heat continuously the discin a pulsating manner at constant frequencies synchronous with the codedata and to hence record the code data in the data area 5. Thisoperation will now be described in more detail with reference to FIG. 2((E) and (F)).

As shown in FIG. 2(E), the laser 23 is continuously oscillated at itslow output by the laser driver 26 in the servo area 4 to readpreformatted pits such as wobbled pits 6 or a clock pit 7. The intensityof the laser is modulated in the data area 5 synchronously with theclock 9 during recording to emit successive high-output light pulses 30narrow in width (for example, 30 ns) to locally heat the thin film 21 atequal intervals in the data area 5 of the segment 3 on the rotating disc1.

Assume now that new code data 31 to be written into the data area 5 is,for example, (101001). The code data 31 is generated from the datagenerator 37 synchronously with the data clock 9. The polarity of themagnetic field produced by the magnetic coil 27 is set to be "+" if thecode is "1", and is set to "-" if the code is "0" to thereby formmagnetic pulses of polarities (+-+--+) 32 (FIG. 2(F)), which are appliedto the areas which are locally heated at equal intervals in the dataarea 5 by the laser pulses 30. As a result, domains 33 corresponding todata (101001) to be newly written are formed in the data area 5. At thistime, as shown in FIG. 3, if the applied magnetic field pulse is widerthan the laser pulse, the deviation of the position of the domains 33 inwhich data is recorded is reduced. The relationship in phase between thelaser pulse 30 and the magnetic field pulse 32 is set such that thelaser pulse 30 is applied under the condition where the magnetic fieldpulse 32 has sufficiently risen or fallen. The adjustment of this phasecan be made by a phase adjuster 38 shown in FIG. 1. In order toreproduce the domains recorded in the data area 5, the laser 23 iscontinuously oscillated at its low output also in the data area 5 as inthe servo area 4. In that case, the magnetic coil 27 is not driven.

While in the above description the power of the laser pulse 30synchronous with the data clock 9 has been described as being constant,the energy of the laser pulse may be changed in accordance with codedata to be recorded. As shown in FIGS. 4A and 4B, when code data "0" isto be recorded in the same direction of magnetization as themagneto-optic medium is initially magnetized (FIGS. 4A and 4B show therecording of the code "0" in the direction of initial magnetization andthe code "1" in the direction opposite to the direction of initialmagnetization. Conversely, the code "0" may be recorded in the directionof initial magnetization and the code "1" may be recorded in thedirection opposite to that of initial magnetization.), the width of thelaser pulse 30 is increased slightly compared to the recording of thecode data "1" in the opposite direction (FIG. 4A), the power of thelaser pulse 30 is increased (FIG. 4B) or both these processes areperformed to eliminate possible unerased recording domains 33. As justdescribed above, in order to change the energy of the laser pulses inaccordance with code data to be recorded, the code data read from thedata generator 37 is required to be delivered to the laser driver 26 tochange the width, level or both of a current pulse delivered to thelaser 23 in accordance with the code data.

Since each domain 33 formed by the present invention is only heatedinstantaneously with a single light pulse 30, the thin film 21 is notoverheated for longer time. Therefore, the formed domain does not becomecrescent but circular, and the magneto-optical reproduced signal is notdelayed in phase and a satisfactory error rate is obtained.

As just described above, according to the present invention, the laserbeam is caused to locally heat the disc at equal intervals with thelaser beam being phase-locked to the frequency of the data channelclock, and the polarity of the magnetic field is modulated with codedata to be newly written to thereby form domains on the disc and henceto overwrite new code data easily.

We claim:
 1. An information recording apparatus comprising:first meansfor applying a magnetic pulse having a polarity corresponding to data tobe recorded on a magneto-optic recording data medium having at leastcontrol data; signal generating means for generating a reproducingsignal corresponding to the control data; clock means for generating aclock signal having a frequency synchronized with a frequency signalobtained from the reproducing signal, the frequency of the clock signalbeing N times the frequency signal, N being a value greater than 1;second means for continuously irradiating optical pulses in synchronismwith the frequency of the clock signal; and third means for generatingthe magnetic pulse by modulating the data synchronized with thefrequency of the clock signal.
 2. An information recording apparatusaccording to claim 1, wherein the recording medium includes a pluralityof data areas, the second means continuously irradiates the opticalpulses at least over an entire respective one of the plurality of dataareas of the recording medium at a frequency in synchronism with thefrequency of the clock signal.
 3. An information recording apparatusaccording to claim 2, wherein the entire respective one of the pluralityof data areas is delimited from the first area having the control dataof the recording medium along the track of the recording medium.
 4. Aninformation recording apparatus according to claim 3, wherein the secondmeans irradiating an optical pulse for each clock signal.
 5. Aninformation recording method comprising the steps of:preparing anoptical recording disk having at least control data along a track;irradiating a light spot on the optical recording disk; obtaining anelectric signal in accordance with light from the control data, theelectric signal being modulated by the control data; generating a timingpulse in accordance with the electric signal; generating a clock signalhaving a frequency synchronized with the timing pulse, the frequency ofthe clock signal being N times the frequency of the timing pulse;continuously irradiating light pulses in synchronism with the frequencyof the clock signal; and modulating a magnetic field by a data signalsynchronized with the frequency of the clock signal so as to form a datadomain by the heating effect of the light pulses and a magnetic field.6. An information recording apparatus comprising:a clock generator forgenerating a clock signal having a frequency synchronized with afrequency signal obtained from control data formed on a magneto-opticdata recording medium; a light source for continuously irradiating lightpulses on the magneto-optic data recording medium in synchronism withthe frequency of the clock signal; a magnet for applying magnetic fieldpulses having a polarity corresponding to data to be recorded on themagneto-optic data recording medium, the magnetic field pulses beingsynchronized with the frequency of the clock signal; and a pulseadjuster for adjusting a phase of the light pulses and the magneticfield pulses.
 7. An information recording apparatus according to claim6, wherein the pulse adjuster shifts the phase of the magnetic fieldpulses.
 8. An information recording method comprising the stepsof:preparing an optical recording disk having at least control data;irradiating a light spot on the optical recording disk; obtaining anelectric signal in accordance with light from the control data, theelectric signal being modulated by the control data; generating a timingpulse in accordance with the electric signal; generating a clock signalhaving a frequency synchronized with the timing pulse; continuouslyirradiating light pulses, the light pulses being irradiated insynchronism with the frequency of the clock signal; modulating amagnetic field by a data signal synchronized with the frequency of theclock signal; and adjusting a phase of the light pulses and the magneticfield so as to form a data domain by the heating effect of the lightpulses and the magnetic field.
 9. An information recording methodaccording to claim 8, wherein the frequency of the clock signal is Ntimes the frequency of the timing pulse, N being a value greater than 1.10. An information recording apparatus comprising:a clock generator forgenerating a clock signal having a frequency synchronized with afrequency signal obtained from control data formed on a magneto-opticdata recording medium; a light source for continuously irradiating lightpulses on the magneto-optic data recording medium in synchronism withthe frequency of the clock signal; and a magnetic for applying magneticfield pulses having a polarity corresponding to data to be recorded onthe magneto-optic data recording medium, the magnetic filed pulses beingsynchronized with the frequency of the clock signal; wherein the lightsource irradiates at least one light pulse before recording the data.